In this study, monodispersed SiO2 nanoparticles with controllable size ranging from 20 to 100 nm have been successfully prepared by using a St?ber sol-gel process. The particle size, distribution, and morphology were examined by dynamic light scattering (DLS) and transmission electron microscopy (TEM). In order to accurately regulate the particle size, the effects of various reaction parameters on the particle size and morphology of SiO2 nanoparticles were systematically investigated. It was found that, to some extent, the SiO2 particle size increased linearly with the increase of ammonia or water content, as well as the decrease of reaction temperature, while nearly kept constant with the increase of tetraethyl orthosilicate (TEOS). Based upon the results, an empirical formula has been developed to model/predict the particle size of the SiO2 nanoparticles over a wide range of various reaction conditions.